CSS Patterns
------------

This document describes the patterns we are using to organize and write
CSS for JupyterLab. JupyterLab is developed using a set of npm packages
that are located in ``packages``. Each of these packages has its own
style, but depend on CSS variables defined in a main theme package.

CSS checklist
~~~~~~~~~~~~~

-  CSS classnames are defined inline in the code. We used to put them as
   all caps file-level ``const``\ s, but we are moving away from that.
-  CSS files for packages are located within the ``style``
   subdirectory and imported into the plugin's ``index.css``.
-  The JupyterLab default CSS variables in the ``theme-light-extension``
   and ``theme-dark-extension`` packages are used to style packages
   where ever possible. Individual packages should not npm-depend on
   these packages though, to enable the theme to be swapped out.
-  Additional public/private CSS variables are defined by plugins
   sparingly and in accordance with the conventions described below.

CSS variables
~~~~~~~~~~~~~

We are using native CSS variables in JupyterLab. This is to enable
dynamic theming of built-in and third party plugins. As of December
2017, CSS variables are supported in the latest stable versions of all
popular browsers, except for IE. If a JupyterLab deployment needs to
support these browsers, a server side CSS preprocessor such as Myth or
cssnext may be used.

Naming of CSS variables
^^^^^^^^^^^^^^^^^^^^^^^

We use the following convention for naming CSS variables:

-  Start all CSS variables with ``--jp-``.
-  Words in the variable name should be lowercase and separated with
   ``-``.
-  The next segment should refer to the component and subcomponent, such
   as ``--jp-notebook-cell-``.
-  The next segment should refer to any state modifiers such as
   ``active``, ``not-active`` or ``focused``:
   ``--jp-notebook-cell-focused``.
-  The final segment will typically be related to a CSS properties, such
   as ``color``, ``font-size`` or ``background``:
   ``--jp-notebook-cell-focused-background``.

Public/private
^^^^^^^^^^^^^^

Some CSS variables in JupyterLab are considered part of our public API.
Others are considered private and should not be used by third party
plugins or themes. The difference between public and private variables
is simple:

-  All private variables begin with ``--jp-private-``
-  All variables without the ``private-`` prefix are public.
-  Public variables should be defined under the ``:root``
   pseudo-selector. This ensures that public CSS variables can be
   inspected under the top-level ``<html>`` tag in the browser's dev
   tools.
-  Where possible, private variables should be defined and scoped under
   an appropriate selector other than ``:root``.

CSS variable usage
^^^^^^^^^^^^^^^^^^

JupyterLab includes a default set of CSS variables in the file:

``packages/theme-light-extension/style/variables.css``

To ensure consistent design in JupyterLab, all built-in and third party
extensions should use these variables in their styles if at all
possible. Documentation about those variables can be found in the
``variables.css`` file itself.

Plugins are free to define additional public and private CSS variables
in their own ``index.css`` file, but should do so sparingly.

Again, we consider the names of the public CSS variables in this package
to be our public API for CSS.

File organization
~~~~~~~~~~~~~~~~~

We are organizing our CSS files in the following manner:

-  Each package in the top-level ``packages`` directory should contain
   any CSS files in a ``style`` subdirectory that are needed to style
   itself.
-  Multiple CSS files may be used and organized as needed, but they
   should be imported into a single ``index.css`` at the top-level of
   the plugin as `import '../style/index.css';`.

CSS class names
~~~~~~~~~~~~~~~

CSS class naming conventions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We have a fairly formal method for naming our CSS classes.

First, CSS class names are associated with TypeScript classes that
extend ``phosphor.Widget``:

The ``.node`` of each such widget should have a CSS class that matches
the name of the TypeScript class:

.. code:: TypeScript



    class MyWidget extends Widget {

      constructor() {
        super();
        this.addClass('jp-MyWidget');
      }

    }

Second, subclasses should have a CSS class for both the parent and
child:

.. code:: TypeScript

    class MyWidgetSubclass extends MyWidget {

      constructor() {
        super(); // Adds `jp-MyWidget`
        this.addClass('jp-MyWidgetSubclass');
      }

    }

In both of these cases, CSS class names with caps-case are reserved for
situations where there is a named TypeScript ``Widget`` subclass. These
classes are a way of a TypeScript class providing a public API for
styling.

Third, children nodes of a ``Widget`` should have a third segment in the
CSS class name that gives a semantic naming of the component, such as:

-  ``jp-MyWidget-toolbar``
-  ``jp-MyWidget-button``
-  ``jp-MyWidget-contentButton``

In general, the parent ``MyWidget`` should add these classes to the
children. This applies when the children are plain DOM nodes or
``Widget`` instances/subclasses themselves. Thus, the general naming of
CSS classes is of the form ``jp-WidgetName-semanticChild``. This enables
the styling of these children in a manner that is independent of the
children implementation or CSS classes they have themselves.

Fourth, some CSS classes are used to modify the state of a widget:

-  ``jp-mod-active``: applied to elements in the active state
-  ``jp-mod-hover``: applied to elements in the hover state
-  ``jp-mod-selected``: applied to elements while selected

Fifth, some CSS classes are used to distinguish different types of a
widget:

-  ``jp-type-separator``: applied to menu items that are separators
-  ``jp-type-directory``: applied to elements in the file browser that
   are directories

Edge cases
^^^^^^^^^^

Over time, we have found that there are some edge cases that these rules
don't fully address. Here, we try to clarify those edge cases.

**When should a parent add a class to children?**

Above, we state that a parent (``MyWidget``), should add CSS classes to
children that indicate the semantic function of the child. Thus, the
``MyWidget`` subclass of ``Widget`` should add ``jp-MyWidget`` to itself
and ``jp-MyWidget-toolbar`` to a toolbar child.

What if the child itself is a ``Widget`` and already has a proper CSS
class name itself, such as ``jp-Toolbar``? Why not use selectors such as
``.jp-MyWidget .jp-Toolbar`` or ``.jp-MyWidget > .jp-Toolbar``?

The reason is that these selectors are dependent on the implementation
of the toolbar having the ``jp-Toolbar`` CSS class. When ``MyWidget``
adds the ``jp-MyWidget-toolbar`` class, it can style the child
independent of its implementation. The other reason to add the
``jp-MyWidget-toolbar`` class is if the DOM structure is highly
recursive, the usual descendant selectors may not be specific to target
only the desired children.

When in doubt, there is little harm done in parents adding selectors to
children.

Commonly used CSS selectors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We use CSS selectors to decide which context menu items to display and what command
to invoke when a keyboard shortcut is used. The following common CSS selectors are 
intended to be used for adding context menu items and keyboard shortcuts.

**CSS classes that target widgets and their children**
-  ``jp-Activity``: applied to elements in the main work area
-  ``jp-Cell``: applied to cells
-  ``jp-CodeCell``: applied to code cells
-  ``jp-CodeConsole``: applied to consoles
-  ``jp-CodeConsole-content``: applied to content panels in consoles
-  ``jp-CodeConsole-promptCell``: applied to active prompt cells in consoles
-  ``jp-DirListing-content``: applied to contents of file browser directory listings
-  ``jp-DirListing-item``: applied to items in file browser directory listings
-  ``jp-FileEditor``: applied to file editors
-  ``jp-ImageViewer``: applied to image viewers
-  ``jp-InputArea-editor``: applied to cell input area editors
-  ``jp-Notebook``: applied to notebooks
-  ``jp-SettingEditor``: applied to setting editors
-  ``jp-SideBar``: applied to sidebars
-  ``jp-Terminal``: applied to terminals
**CSS classes that describe the state of a widget**
-  ``jp-mod-current``: applied to elements on the current document only
-  ``jp-mod-completer-enabled``: applied to ediors that can host a completer
-  ``jp-mod-commandMode``: applied to a notebook in command mode
-  ``jp-mod-editMode``: applied to a notebook in edit mode
-  ``jp-mod-has-primary-selection``: applied to editors that have a primary selection
-  ``jp-mod-in-leading-whitespace``: applied to editors that have a selection within the beginning whitespace of a line
-  ``jp-mod-tooltip``: applied to the body when a tooltip exists on the page
**CSS selectors that target data attributes**
-  ``[data-jp-code-runner]``: applied to widgets that can run code
-  ``[data-jp-interaction-mode="terminal"]``: applied when a code console is in terminal mode
-  ``[data-jp-interaction-mode="notebook"]``: applied when a code console is in notebook mode
-  ``[data-jp-isdir]``: applied to describe whether file browser items are directories
-  ``[data-jp-undoer]``: applied to widgets that can undo
-  ``[data-type]``: applied to describe the type of element, such as "document-title", "submenu", "inline"
